Manganese dioxide electrodes as used in rechargeable alkaline manganese dioxide cells are reversible only if the manganese dioxide cathodes are discharged to the point where the MnO.sub.2 is converted to Mn.sub.2 O.sub.3 (i.e., the one electron discharge level). It has been well established that if the discharge continues beyond that level, an irreversible phase change occurs so that the manganese dioxide cathode is no longer rechargeable. Under certain conditions, it is now possible that MnO.sub.2 electrodes for rechargeable alkaline cells can be rendered reversible within the two electron range.
It has always been desirable to produce rechargeable manganese dioxide cells with zinc anodes--especially cylindrical cells having conventional cylindrical configurations--to have high energy density. This has not been particularly successful, and numerous difficulties have been encountered. Several approaches have been provided to ensure reversibility and rechargeability of MnO.sub.2 cells, including the provision of electronic means so as to prevent overdischarge of the MnO.sub.2 ; designing the cell so as to be anode limited; and modifying the MnO.sub.2 particularly by the addition of heavy metals thereto.
Indeed, rechargeable alkaline MnO.sub.2 /Zn cells have been available, at least in the North American market, since the late 1960's. However, those cells were not generally successful, and by the mid 1970's they were removed from the market. At least in part, the lack of success of those cells was due to the fact that they were generally assembled in batteries and not available in single cell configurations, and that they were required to be monitored very carefully to determine the end of the useful discharge capacity. Such monitoring was by timing the operation of the cells, or by the expensive incorporation of electronic control means to determine the point of discharge beyond which further discharge could not be tolerated. Moreover, the cells were quite modest in terms of their density capabilities, and "D" cells having nominal 2 Ah capacities had energy densities of, at best, about 52 Wh/liter or 18 Wh/Kg.
To overcome those difficulties, cells were then developed by which anode limitation of the cells was imposed; meaning that the capacity of the anode was severely limited so that it became impossible to discharge the manganese dioxide cathodes to more than about 40% of their theoretical capacity. By these means, the rechargeability of the MnO.sub.2 was assured. By providing cells having severe anode limitation characteristics, however, the cells were thereby prejudiced by having quite low energy densities, and therefore the cells were not widely accepted in the market.
Several specific patents which address some of the issues above, and the approaches to preserve the rechargeability of cells, are discussed below:
AMANO et al, U.S. Pat. No. 3,530,496, issued Sep. 22, 1970, provide alkaline MnO.sub.2 /Zn cells which are rechargeable, but where the depth of discharge of the manganese dioxide cathode is severely regulated by limiting the available capacity of the zinc anode to less than 40% of the available capacity of the MnO.sub.2 cathode. Indeed, Amano et al suggest that anode limitation is preferably in the range of 20% to 30% of the MnO.sub.2 cathode capacity, to achieve optimum performance of the cells. In practise, the theoretical capacity of the cell is not realized, except at very low drain rates.
The Amano et al cells achieve their zinc anode limitations by providing cathodes which are essentially equal in their dimensions to those of primary alkaline cells, and then reducing the zinc capacity by placing a cylindrical gelled zinc anode adjacent to the MnO.sub.2 cathode and separated from it by suitable two component separators; and then by filling the center of the cell with gelled electrolyte that does not have any active anode material added to it. It should be noted that Amano et al prefer amalgamated copper particles to be included in the anode so as to enhance its conductivity. They also provide a zinc oxide reserve mass, and they must use a perforated coated screen current collector rather than a single nail as might otherwise be used in primary alkaline cells--and as used in the present invention.
AMANO et al teach that the stoppage of discharge of the rechargeable alkaline manganese cells is regulated by the capacity of the zinc negative electrode, so that the discharge depth of the positive electrode is controlled. This precludes the necessity to interrupt discharge of the cell either as a consequence of voltage or time, and thus overcomes some of the difficulties experienced with earlier cells. The cell is described as having a positive electrode made of pressurized mixture of MnO.sub.2 together with graphite powder, etc., and that it can be used within 20% of the real capacity of a similar primary cell. The cell may provide 40 to 60 cycles however after 40 to 60 charge-discharge cycles there is no reversibility of the positive electrode.
KORDESCH, in U.S. Pat. No. 4,091,178, issued May 23, 1978, provides a rechargeable MnO.sub.2 /Zn in which the anode capacity is specifically limited to about 33% of the capacity of the cathode. Kordesch also provides a charge reserve mass in which a quantity of zinc oxide is placed equal to at least 50% of the anode discharge capacity. Once again, because there is an excessive capacity of MnO.sub.2 as well as additional ZnO, the energy density of the Kordesch cell is quite low.
DZIECIUCH et al, were granted U.S. Pat. No. 4,451,543 on May 29, 1984. That patent teaches a rechargeable MnO.sub.2 /Zn cell where the MnO.sub.2 is doped with heavy metals such as bismuth or lead. The intention is that up to 50% of the theoretical two electron capacity of the MnO.sub.2 can be reached. However, the MnO.sub.2 cathode comprises relatively high quantities of carbon, which results in the cathodes having a low specific density and a low cell energy density. Still further, it must be noted that in practical cells the second electron reduction step of the MnO.sub.2 occurs at a too low voltage, namely below 0.9 volts. It is questioned, therefore, whether such cells as are provided by Dzieciuch et al are capable of delivering even a relatively substantial portion of their theoretical capacity above 1.1 volts--the minimum operating voltage of various electronic devices--or even above the 0.9 volt cutoff voltage that is generally required in uses such as toys, small battery driven appliances, and the like.
OGAWA et al, in U.S. Pat. No. 3,716,411, issued Feb. 13, 1973, teach a rechargeable alkaline manganese cell, the discharge capacity of the anode of which is controlled within such a range that the cathode can be recharged; and wherein the anode and cathode face each other through a gas permeable and dendrite impermeable separator. However, the OGAWA et al cell is strictly anode limited in that the capacity of the anode is held to be not more than about 40% of the theoretical one electron discharge capacity of the manganese dioxide. OGAWA et al discuss the fact that if a zinc-manganese dioxide cell is discharged so that its terminal voltage reaches a voltage below 0.9 volts and down to about 0.75 volts, and where the capacity of the zinc negative electrode is about the same or slightly smaller than that of the manganese dioxide positive electrode, then the effect of the discharge on the manganese dioxide is such that it is non-reversible at least in part. OGAWA et al provide that under no conditions should the depth of discharge of the anode be permitted to exceed 60% of the theoretical one electron discharge capacity of the manganese dioxide cathode. OGAWA et al provide an alternative structure which comprises two positive electrodes, one on either side of the anode, and wherein the inner positive electrode is contained within a perforated nickel plate steel pocket or canister.
KORDESCH et al, U.S. Pat. No. 4,957,827 issued Sep. 18, 1990, is commonly owned herewith. KORDESCH et al teach a cathode which is preferably molded having a plurality of discrete pellets. However, it must be noted that KORDESCH et al require the use of a screen as an oxygen evolution catalyst. Moreover, the anode capacity of the KORDESCH et al patent is limited to about 40% of the one electron capacity of the MnO.sub.2 cathode--in other words, having a similar capacity limitation to AMANO et al described above.
Applicant also refers to OGINO et al U.S. Pat. No. 4,863,817, issued Sep. 5, 1989. That patent is directed to a cell which uses electrolytic manganese dioxide in its cathode, but OGINO et al specifically describe a cell that has a non-aqueous electrolyte (e.g., having a lithium electrode). OGINO et al achieve the electrolytic MnO.sub.2 by subjecting a manganese dioxide-forming electrolytic solution to electrolysis which introducing a gas such as nitrogen through a plating solution. OGINO et al specifically require high purity manganese dioxide to obtain the excellent charge and re-charge properties with discharge voltage flatness, relatively high capacity and better cycle performance.
The matter of capacity utilization in primary alkaline cells which have an additional amount of manganese dioxide is addressed in LEGER U.S. Pat. No. 2,993,947, issued Jul. 25, 1961. There, the additional MnO.sub.2 is found in the cell so as to preclude the cell from leaking if it is left in an operating device and is thereby utilized as an energy source over extended periods of time. It must not be overlooked, however, that LEGER contemplates only primary cells.
Clearly, the intent of the present invention is to provide cells having a high initial capacity, high discharge voltage, high cumulative capacity, an extended cycle life, and cells that are capable of maintaining high drain rates over most of their lifetime. Moreover, cells according to the present invention must be capable of being easily and economically manufactured, with production costs substantially in the order of the production costs of high quality primary alkaline cells.